Repair of corroded reinforcement in concrete using sacrificial anodes

ABSTRACT

Reinforcement in concrete is cathodically protected by galvanically connecting a sacrificial anode, such as a zinc or zinc alloy anode, to the reinforcement, and contacting the anode with an electrolyte solution having a pH which is maintained sufficiently high for corrosion of the anode to occur, and for passive film formation on the anode to be avoided. The pH of the electrolyte is preferably at least 0.2 units, and preferably from 0.5 units to more than 1.0 units, above the pH value at which passivity of the anode would occur. The electrolyte may be for example sodium hydroxide or potassium hydroxide but is preferably lithium hydroxide which also acts as an alkali-silica reaction inhibitor.

BACKGROUND OF THE INVENTION

This application is the national phase of international applicationPCT/GB94/01224 filed June 6, 1994 which designated the U.S.

This invention relates to the cathodic protection of reinforcedconcrete.

The application of cathodic protection to steel reinforcement inconcrete is an accepted method of providing corrosion protection for themetal, particularly where chloride ions are present at significantconcentrations in the concrete.

Cathodic protection involves the formation of a circuit with thereinforcement acting as a cathode, electrically connected to an anode,with the circuit being completed by pore solution in the concrete and anelectrolyte contacting the anode. When a potential difference existscorrosion of the cathode is prevented or reduced.

It is known to create a potential difference between an anode and acathode both by means of impressed current cathodic protection whichinvolves the use of a non-sacrificial anode and an applied electriccurrent using an external DC power supply and by means of a galvaniccell in which the potential arises as a result of the differentmaterials forming a sacrificial anode and a cathode.

Where a galvanic cell is used it is important that the electrolytecontacting the anode is such that sustained active corrosion of theanode can occur. If suitable conditions are not maintained then thecathodic protection will become inefficient.

Furthermore, the electrolyte must be such that its contact with thesurrounding concrete does not result in the degradation of the concrete.Of particular significance in this context is the susceptibility of someaggregates, present in concrete, to alkali-silica or ine the aggregatereactions. These reactions can cause swelling and consequential crackingof concrete.

SUMMARY OF THE INVENTION

According to the invention there is provided a method of cathodicallyprotecting reinforcement in concrete in which a sacrificial anode isgalvanically connected to the reinforcement. The anode is contacted withan electrolyte solution having a pH which is maintained sufficientlyhigh for corrosion of the anode to occur and for passive film formationon the anode to be avoided.

According to a further feature of the invention there is provided a unitfor use in the cathodic protection of reinforcement in concrete whereinthe unit comprises a sacrificial anode in contact with a materialcontaining an electrolyte which in solution has a pH which issufficiently high for corrosion of the anode to occur and for passivefilm formation on the anode to be avoided when the anode is galvanicallyconnected to the reinforcement.

According to yet a further feature of the invention there is provided anarticle of reinforced concrete wherein the reinforcement is cathodicallyprotected by the method described above.

To avoid passivation of the anode a suitable pH must be maintainedaround the anode. Although for zinc a suitable pH value is >13.3, orpossible >13.5, and preferably >14, other materials when used as theanode may require other electrolyte pH limits to avoid passivity. Inpractice while any pH above the "boundary value" at which passivity islikely may be suitable in the short term, it is advantageous to have apH well above the "boundary value" to start with. During cathodicprotection the pH near the anode is likely to drop and so a higherinitial pH acts as a reserve to maintain activity over a long period. pHvalues of 0.2 above the "boundary pH" may be acceptable, but pH values,0.5, 0.7 and 1.0 or more units above the "boundary pH" are likely togive a better reserve and a better long term performance.

The anode material selected will determine the electrolyte pH requiredto maintain active corrosion. In general terms the material chosen mustbe more reactive, and preferably significantly more reactive, than thematerial forming the reinforcement.

The anode is preferably zinc or a zinc alloy but the anode may bealuminium, an aluminium alloy, cadmium, a cadmium alloy, magnesium or amagnesium alloy or another material which has a more negative standardelectrode potential than the reinforcement under the prevalentconditions.

The electrolyte may be for example sodium hydroxide or potassiumhydroxide.

Advantageously, in some circumstances, at least one alkali-silicareaction inhibitor is also present, in at least a portion of theelectrolyte.

The high pH of the electrolyte may be due, at least in part, to one ormore of the alkali-silica reaction inhibitors.

Preferably at least one of the alkali-silica reaction inhibitors isprovided in an hydroxide form. Most preferably the, or one of theinhibitors is lithium hydroxide, which can also function as theelectrolyte itself.

The electrolyte solution may be the pore solution of the concrete and/orthe pore solution of a mortar, paste or other porous material applied tothe concrete being protected.

The method may be practiced during the course of repairing reinforcedconcrete by connecting one or more sacrificial anodes to thereinforcement and applying repair material and the electrolyte to theare pair site.

Preferably the anodes are provided in the vicinity of the repair site.If the anode is provided away from the repair site there is likely to bea loss of efficiency due to the extra circuit length required tocomplete the galvanic cell. Most preferably the anodes are provided nearthe periphery of the repair site. The anodes are preferably in the newmaterial of the repair site. There may be many anodes. The anode oranodes may have a relatively large surface area and for example could bea mesh or wire (or wires) extending adjacent to the periphery of therepair site.

Preferably each anode is substantially enclosed in repair materialcontaining an electrolyte of high pH. The portion of repair materialaway from the anode may have a different pH compared with the portion ofrepair material substantially enclosing the anode. The repair materialaway from the anode may have a pH that is relatively moderate or lowcompared with that near the anode.

The whole or any portion of the repair material may also contain one ormore alkali-silica reaction inhibitors.

Where only a portion of the material contains an electrolyte of high pHand only a portion contains one or more alkali-silica reactioninhibitors the portions may be the same, distinct or overlapping inextent.

Preferably at least one of the alkali-silica reaction inhibitors alsocontributes to the high pH of the electrolyte.

As well as introducing sacrificial anodes and an electrolyte of high pHto a structure during a repair, potentially along with an alkali-silicareaction inhibitor, this invention is also applicable to theconstruction of new reinforced concrete articles or structures and tothe improved protection of existing ones.

Just as during repair, anodes and a suitable electrolyte can be providedin electrical contact with the reinforcement to form a galvanic cell, soa similar arrangement can be generated during construction.

The entire structure can be provided with a suitable electrolyte, ormerely that portion in the vicinity of the anode can be so provided.

In the construction of new reinforced concrete articles or structuresone or more sacrificial anodes can be connected to the reinforcement, amaterial containing the electrolyte cast around the anode or anodes andconcrete then cast around the electrolyte-containing material.

In the improvement of the protection of existing concrete articles oneor more sacrificial anodes can be inserted in a hole in a mass ofreinforced hardened concrete and connected to the reinforcement and thensurrounded by a material containing the electrolyte.

In both methods the material containing the electrolyte can be anon-cementitious material or a cementitious material.

One or more of the sacrificial anodes may be introduced to the repairsite as a pre-formed unit comprising an anode in contact in use with aporous material containing an electrolyte of high pH. The material mayalso contain one or more alkali-silica reaction inhibitors. The unit mayhave an anode substantially enclosed in porous material of high pH.

The sacrificial anode may be at least partially enclosed in thematerial. Only a portion of the material which contacts the anode maycontain an electrolyte of high pH. Of course more than one anode couldbe provided in each unit.

The unit may comprise a container holding the material and the anode.The unit may be ready for introduction to a repair site, or may requiresome local treatment (for example wetting). The unit may comprise a bagor sock which contains the high pH material and an anode.

It is possible to provide a localized area of high pH adjacent or aroundthe anode and this will probably occur in patch repair automatically.However it may be desirable to have a region of higher pH even whenmaking original concrete articles.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows a repair site in a reinforced concrete article, with thereinforcement exposed and sacrificial anodes attached; and

FIG. 2 shows a cross section through a reinforced concrete article witha sacrificial anode unit embedded therein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Chloride-contamination of concrete structures can cause significantcorrosion in reinforced structures. Such corrosion is often localizedand can cause cracking of concrete surrounding the reinforcement. It isnormal to treat problems of local corrosion-induced cracking inreinforced concrete structures primarily by removing the affectedmaterial and patching with fresh cementitious mortars or concretes. Acommon difficulty which arises in such cases is that failure to detectand remove all chloride-contaminated concrete from around thecorrosion-damaged areas can result in the formation of so-called"incipient anodes"on the reinforcing steel in the vicinity of the repairpatches, which are electrically coupled to cathodic steel situated inthe repaired areas themselves. This can lead to rapid corrosion at the"incipient anodes" and to eventual cracking of the concrete around therepaired areas.

However, if having removed the contaminated and cracked concrete fromaround the reinforcement in regions of the structure where corrosion hasbeen detected or where chloride salts have been found in significantconcentrations, the exposed steel is cleaned and connected to zinc-basedsacrificial anodes at locations near the periphery of the area to bepatched and the repair site is reinstated with mortar (or a similarmaterial) of suitably controlled high pore solution pH, (for examplepH >13.3, 13.5 or 14 for zinc or zinc alloy anodes) such problems can beovercome.

FIG. 1 illustrates such a repair where a contaminated volume of concretehas been removed from a concrete slab 1 to leave a void. As a result thereinforcement 2 is exposed. The reinforcement 2 can then be cleaned anda series of zinc anodes 3 can be attached by connectors 4 to thereinforcement at locations 5. The anodes may conveniently be locatedaround the periphery of the area to be protected.

Subsequently repair mortar can be applied to fill the void.

The pore solution of the repair mortar acts as the electrolyte tocomplete the circuit enabling cathodic protection to take place, withthe high pH ensuring that corrosion of the anode and hence theprotection is sustainable.

In many cases a pore solution having pH values high enough for use inthe above applications may be made either from Portland cements ofintrinsically high alkali content (i.e. those containing relatively highproportions of Na₂ O and K₂ O or from cements of lower alkali contentwith supplementary alkalis (in the form of LiOH, NaOH or KOH forinstance) incorporated into the mix materials as admixtures.

In some instances, because the presence of high concentrations ofhydroxyl ions in combination with sodium and potassium ions can causealkali-silica reactions, which can cause deleterious expansion andcracking of the concrete, the presence of an alkali-silica reactioninhibitor is advisable.

Where a potentially reactive aggregate is present the mortar can be madefrom a cement of relatively low alkali content with lithium hydroxide asan admixture. Typically, this would involve the addition of LiOH to themix water at a concentration of about 1 mole/liter or higher, whichwould ensure the maintenance of a high pH value, necessary to sustainthe activity of the zinc-based anode, whilst introducing a cation, Li⁺,that is known to act as an inhibitor of alkali-silica reaction.

The use of lithium hydroxide as admixture is of especial benefit whenthe mortar, concrete, or the like, has a low Na and K content (or a lowNa or K content). Li⁺ can assist in preventing alkali aggregatereaction.

Alternatively other inhibitors may b e added to the material in use, forexample to the mix water, in conjunction with a pH adjusting reagent.

The inhibition effect of such reagents is aided further in that thecurrent resulting from the cathodic protection encourages migration ofthe inhibitor to the preferential alkali-silica reaction sites, (wherethe inhibitor has a positive charge, as is the case for lithium ions).Thus lithium ions migrate over time and there is in use a higherconcentration of t hem where they are desirable.

As an alternative to (or in addition to) using repair mortars (orsimilar) of high pH value to reinstate the entire region of removedconcrete, it is also possible to utilize sacrificial zinc-based anodeswhich have been precast in mortars of suitable composition. Such anarrangement is shown in FIG. 2, where a sacrificial anode 6 is almostentirely enclosed in a block of precast mortar 7 to form a discreteunit. A connector 8 allows connection of the anode to the reinforcement9 in use. The mortar 7 contains an electrolyte of a sufficient pH toensure that the anode remains active, in use.

Having galvanically coupled the anode to the steel reinforcementreinstatement of the regions to be patched may then be carried out withmortar or concrete 10 of moderate or low alkali content because thesacrificial anode 6 has already been surrounded by mortar 7 containingan electrolyte that will sustain its activity, allowing effectivecathodic protection of the steel. Surrounding the anode with high pHmortar is preferred, but it may not be essential to surround it fully.

As well as precast units the provision of porous bags or sockscontaining an anode and the mixtures for the mortar is envisaged. Thehigh pH electrolyte, with or without alkali-silica reaction inhibitors,may then be added at the location of the structure in question. Otherporous material to enclose the anode, for example foams, plastics,sponges are also envisaged.

As a further alternative it is possible to apply the anode as a coatingor layer (for example as a paint to the reinforcement). It is usuallydesirable to clean the reinforcement first in such applications. Thepaint would be rich in the dissimilar metal or composition which formsthe anode, so providing cathodic protection in that way. Zinc or zincalloys are particularly suitable for such applications.

To ensure the continued activity of the anode the electrolytesurrounding the reinforcement applied paint needs to be at a high pH. Asuitable pH for zinc is >14 although pH values >13.3 are believed towork for at least a limited period. However, the remainder of the repairmaterial could once again be of lower or more moderate pH (or could beof the same pH).

If the concrete was judged to be susceptible to alkali-silica reactionsthen lithium ions or other inhibitors could be provided either in thesurrounding electrolyte (or in the paint or coating forming the anode).If the concrete were judged not to be susceptible to alkali-silicareactions then it may be preferable to use NaOH or KOH (or some otheralkali) to produce the high pH rather than lithium hydroxide.

When treating an existing structure without the need for repair,sacrificial anodes can be provided in proximity with a surface of thestructure. Mortar, paste or other porous material containing a suitablyhigh pH electrolyte can be introduced to connect the anode to the poresolution of the existing concrete; with the anode connected to thereinforcement to complete the circuit. Alkali-silica reaction inhibitorscan also be introduced to the electrolyte and so can migrate into theexisting structure because of the galvanic potential.

Instead of providing the anodes in their own pre-cast high pHenvironment (with or without the presence of alkali-silica reactioninhibitors) it is possible to apply a region of high pH (and/oralkali-silica reaction inhibitors) mortar in the vicinity of the or eachanode, and a region of different pH elsewhere (for example as an upperlayer on top of a lower layer). The anode would still be in contact witha high pH electrolyte.

The ready made anode unit shown in FIG. 2 comprises a pre-cast concreteblock. However other units may be provided, such as for example bags orsocks of high pH concrete or mortar which also contain an anode which isin use connected to the reinforcement. The bags may be provided withwet, unset, mortar, or may be provided dry, the user wetting them beforeuse. They may even in some unlikely circumstances be used dry, absorbingthe necessary liquid from their surroundings (when they are cast intoplace). The units would normally also contain a connector to connect theanode to the reinforcement. The anodes may be provided separately fromthe bags of high pH material and introduced to the high pH material uponinstallation.

It will also be appreciated that the high pH material in contact with(and preferably surrounding) the anode need not be mortar or concrete,so long as it is permeable to the electrolyte. It preferably has goodmechanical strength in use, but not necessarily. In an extreme case itcould be spongy.

It will be appreciated that the pH of the concrete, mortar or the likeis controlled either by choosing the composition of the repair materialso as to give a suitably high pH, or by deliberately adding admixtures(such as KOH and/or LiOH, and/or NaOH) to give the desired pH. Thuscontrolling the pH is a step in the method.

The following example will serve to further illustrate the invention.

Mild steel bars 6 mm in diameter were cut into 80 mm lengths, cleanedusing 600 grade carbide paper, degreased in acetone and stored in adesiccator for a minimum of 2 days so that a uniform oxide film coulddevelop on the surface. The two ends of the steel specimens were maskedusing a styrene-butadiene rubber modified cement slurry and epoxy resinin such a way as to expose a 10 cm² area of the central region of eachspecimen. The top 3 mm of each specimen was left unmasked to provide anelectrical connection during monitoring. These mild steel specimens wereindividually fixed in a hole on the lids of cylindrical PVC containers(45 mm dia., 75 mm high). Similarly, strips of zinc 1 mm thick 10 mmwide and 80 mm long were prepared in the same way allowing a centralregion of 10 cm² to be exposed. These strips were also fixedindividually on lids.

Duplicate cement pastes of a 0.5 water/cement ratio and containing 3%chloride by weight of cement as sodium chloride were then produced. Thefreshly made mix was emptied into the PVC containers in two stages,vibrating after each stage. The lids containing the steel electrodeswere then fixed on to the containers and after further vibration of afew seconds for compaction, the cast specimens were allowed to stand for24 hours in ambient conditions. After demolding, the specimens werestored in a 100% relative humidity environment at room temperature. Thecement was an ordinary Portland cement of about 0.6% alkali contentexpressed as Na₂ O equivalent. This level of alkali produced a cementpaste whose pore-solution had a pH of about 13.6. In the same way thezinc electrodes were embedded in cement pastes containing 0 or 2 molarNaOH or LiOH dissolved in the mix water. Such additions of alkalihydroxides raised the pH of the pore-solution to a level higher than 14.

The corrosion potential of each individual steel or zinc electrode wasmeasured regularly with a voltmeter against a standard saturated calomelelectrode rested on a damp piece of tissue paper positioned on each ofthe cement paste specimens. After three weeks, one of the steelelectrodes and one zinc specimen containing 2 molar NaOH were positionedside by side at a distance of around 5 cm in a container able tomaintain a near 100% relative humidity and whose base was lined with wettissue paper. The two electrodes were electrically connected so that acurrent could pass between them.

The potential of the corroding steel embedded in chloride contaminatedcement paste quickly fell to a value lower than -400 mV and oscillatedaround this value throughout the exposure period of over 300 days. Thepotential of the zinc electrode embedded in cement paste without anyadditions after starting at a very negative potential of around -750 mVgradually climbed to more noble potentials of around -400 mV. Thesimilarity of the potential of the two sets of electrodes will restrictthe flow of current between them when coupled and protection of thesteel against corrosion would be unlikely. Such protection will only beachieved if a significant potential gradient existed between the twometals. The addition of 2 molar sodium hydroxide or lithium hydroxidewas able to bring the potential of the zinc to potentials of around -700mV, values significantly lower than those obtained for the corrodingsteel. The coupling of the steel electrode with the zinc embedded in apaste whose alkalinity was enhanced by addition of NaOH resulted in agalvanic current which eventually stabilized at around 2.5 μA (0.25μA/cm² or 2.5 mA/m² of steel area) a level of current normally appliedin cathodic protection systems in steel reinforced concrete. The"instant off" potentials of the steel and the zinc electrodes after 275days were -426 mV and -640 mV respectively. The potential of the steelafter 24 hours of disconnection rose to a very noble value of -207 mVcompared to -470 mV of the parallel unprotected steel specimen,indicating a substantial degree of protection of the steel by the zincanode.

I claim:
 1. A method of cathodically protecting steel reinforcement inconcrete, comprising the steps of:(a) galvanically connecting asacrificial anode to the steel reinforcement, the anode being zinc or azinc alloy so as to have a more negative electrode potential than thatof the steel reinforcement; and (b) casting around the anode a porousmaterial containing an electrolyte solution with sufficient alkali thatits pH is at least about 14 so as to be above a pH at which passivity ofthe anode would occur, whereby corrosion of the anode and substantialprotection of the steel reinforcement are maintained and passive filmformation on the anode is avoided.
 2. A method as recited in claim 1,wherein at least one alkali-silica reaction inhibitor is present in theelectrolyte.
 3. A method as claimed in claim 2, wherein the inhibitor isa source of lithium ions.
 4. A method as claimed in claim 3, wherein theinhibitor is lithium hydroxide.
 5. A method as recited in claim 1,wherein step (b) is practiced by casting a material containing theelectrolyte solution about the anode and casting concrete around theelectrolyte-containing material so that the anode is embedded in theconcrete and is substantially surrounded by the electrolyte solution. 6.A method as recited in claim 5, wherein step (a) is practiced byinserting the sacrificial anode in a hole in a mass of hardened concreteand connected to the reinforcement.
 7. A method as recited in claim 1,wherein step (b) is practiced so that the material containing theelectrolyte is a cementitious material.
 8. A method as recited in claim1, comprising repairing corrosion-induced cracked reinforced concrete bythe steps of:(c) removing the corrosion-induced cracked concrete toexpose the steel reinforcement; (d) cleaning the reinforcement; and (e)connecting sacrificial anode to the cleaned reinforcement.
 9. A methodas recited in claim 8, wherein the concrete is chloride contaminated,and wherein step (c) is practiced by removing all the chloridecontaminated concrete.
 10. A method as claimed in claim 1, comprisinginserting an anode in a hole in a mass of hardened concrete and castingthe porous material containing the electrolyte around the anode.
 11. Aunit for use in the cathodic protection of steel reinforcement inconcrete, the unit comprising:a sacrificial anode for embedding in theconcrete and for connecting to the reinforcement, the anode being zincor a zinc alloy so as to have a more negative electrode potential thanthat of the steel reinforcement; and a repair material for repairingconcrete cast around the anode such that the anode is substantiallyenclosed in the repair material, the repair material containing anelectrolyte having a pH which is at least about 14 so as to be above apH at which passive film formation on the anode would occur when theanode is galvanically connected to the steel reinforcement, wherebycorrosion of the anode and substantial protection of the steelreinforcement are maintained and passive film formation on the anode isavoided.
 12. A unit as recited in claim 11, wherein the anode isenclosed in a block of precast concrete or mortar containing theelectrolyte and the anode has a connector for connection to thereinforcement.
 13. A unit as recited in claim 11, wherein the repairmaterial is subjected to a wetting preliminary treatment before the unitis embedded in the concrete, and wherein the entire unit is embedded inthe concrete.
 14. A unit as recited in claim 11 wherein the repairmaterial is cementitious.
 15. A repair kit for corrosion-induced crackedsteel reinforced concrete, the repair kit comprising:a container; asacrificial anode for embedding in the concrete and for connecting tothe metal reinforcement, the anode being zinc or a zinc alloy so as tohave a more negative electrode potential than that of the steelreinforcement; and a repair material for concrete for contacting theanode and containing an electrolyte having a pH which is at least about14, and thereby above the pH value at which passivity of the anode wouldoccurs when the anode is galvanically connected to the steelreinforcement, and whereby corrosion of the anode and substantialprotection of the steel reinforcement is maintained, and passive filmformation on the anode is avoided; and wherein said sacrificial anodeand said repair material are disposed in the container.
 16. A repair kitas recited in claim 15, wherein the container in which the anode and therepair material containing the electrolyte are disposed is a bag orsock.